FULL CREDIT to BRUCE MOMJIAN for this .http://momjian.us/
To prevent Firefox from bugging you to upgrade from Foxmarks 2.7.2. Open “about:config”, right-click to create a new
boolean variable, extensions.foxmarks@kei.com.update.enabled, and set it to ‘false’; you will then no longer be prompted to upgrade
Foxmarks. Download 2.7.2 by clicking on “See All Versions” from
addons.mozilla.org.

To go back to old version do the following.
1. Note your Xmarks username and password.
2. Uninstall Xmarks.
3. Go to https://addons.mozilla.org/en-US/fire....7.2
4. Install Foxmarks.
5. Login with your username and password.